Imaging element, driving method, and electronic device

ABSTRACT

The present technology relates to an imaging element and a driving method, and an electronic device that enable stable driving with low voltage and low power consumption and furthermore make it possible to ensure a time resolution of detection. A light detector includes a pixel array section including a plurality of first pixels and a second pixel. The first pixel includes a photoelectric conversion section that photoelectrically converts incident light, a floating diffusion section that generates a voltage in accordance with the amount of charge carriers obtained by photoelectric conversion, and a transfer section that transfers charge carriers from the photoelectric conversion section to the floating diffusion section; the readout of a signal is performed intermittently from the first pixel. Further, the output of the second pixel is monitored continuously to detect the incidence of light. The present technology can be applied to a radiation counter.

TECHNICAL FIELD

The present technology relates to an imaging element and a drivingmethod, and an electronic device, and relates particularly to an imagingelement and a driving method, and an electronic device that enablestable driving with low voltage and low power consumption andfurthermore make it possible to ensure a time resolution of detection.

BACKGROUND ART

Radiation counting (photon counting) that counts the dose of radiationincident on a detector while performing an individual energy separationon an incident photon basis is applied to various fields at present,such as survey meters, positron emission tomography (PET), and gammacameras.

Usually a scintillator and a photomultiplier are used as the detector,and the energy and number of radiation carriers incident on the detectorare counted.

If one or more photons of radiation enter the scintillator, thescintillator emits light, and releases a pulse of an amount of visiblelight in proportion to the energy of radiation (hereinafter,occasionally referred to as a light emission pulse).

Such a light emission pulse is emitted each time a radiation photonenters the scintillator, and is sensed by the photomultiplier.

Here, the scintillator is covered with a separating wall in which only asurface facing the photomultiplier is set in an open state. Theseparating wall blocks the entry of visible light from the outside, andpreferably reflects light generated from the inside and causes all thelight to be incident on the photomultiplier.

In a radiation counter including a scintillator and a photomultiplier,the photomultiplier converts a light emission pulse to electrons, andamplifies the electrons to generate an analog electrical pulse.

The pulse height of the analog electrical pulse is in proportion to theamount of emitted light of the scintillator, that is, the energy ofradiation. Then, an independent pulse is outputted each time oneradiation photon is incident; thus, the radiation counter can find thenumber of incident radiation photons by counting the number of pulses.

In the radiation counter described above, a detection circuit amplifiesand shapes a pulse to change the pulse to an analog wave having amoderate delay, and converts the analog wave to a digital value with ananalog-to-digital (A/D) converter. Thereby, the radiation counter canderive energy for each incident radiation photon, in a digital value.

A digital processing circuit in the radiation counter accumulates outputresults of the detection circuit obtained in a prescribed period oftime, and derives an energy spectrum of radiation photons. This showsthe existence ratio on an energy basis of the radiation photons capturedby the radiation counter. Thereby, the radiation counter can identifythe radiation source, and can separate radiation that is incidentdirectly from a known radiation source and radiation that is scatteredand has lost some energy on the way.

Further, as a radiation counter, also a radiation counter in which ascintillator and an imaging element are combined is proposed (forexample, see Patent Literature 1). In this radiation counter, theimaging element is used as a detector of radiation, and the energy andnumber of radiation carriers are counted.

CITATION LIST Patent Literature

Patent Literature 1: JP 2015-76773A

DISCLOSURE OF INVENTION Technical Problem

Meanwhile, for radiation counters like those described above, one usinga scintillator and a photomultiplier is the mainstream. However, thephotomultiplier is expensive, and furthermore is not suitable for sizeand weight reduction and has the property of being susceptible tomagnetic fields.

Thus, also a radiation counter that uses, in place of thephotomultiplier, an array of avalanche photodiodes (APDs) or siliconphotomultipliers (SiPMs), or the imaging element described above isproposed.

However, in a radiation counter using APDs, the output signal is veryfeeble, and furthermore there is a significant output variation due totemperature; hence, this radiation counter is likely to be affected bythe external environment. Further, in a radiation counter using SiPMs, ahigh electric field is required and hence the dark current is large, andthe floor noise is large due to after-pulses, crosstalk, etc.

Furthermore, both in the APD and the SiPM, a high voltage is used andhence a power supply circuit is needed additionally, and the output isan analog signal. Hence, it is necessary that an amplifier, anintegrating circuit, and an A/D conversion circuit be externallyattached additionally, and the counter is likely to be affected byexternal noise during the course of signal transmission.

Further, in a radiation counter using an imaging element, it has beendifficult to ensure a sufficient time resolution for the detection of alight emission pulse generated by incident radiation.

The present technology has been made in view of such circumstances, andenables stable driving with low voltage and low power consumption andfurthermore makes it possible to ensure a time resolution of detection.

Solution to Problem

According to a first aspect of the present technology, an imagingelement includes: a pixel array section including a plurality of firstpixels each including a first photoelectric conversion sectionconfigured to photoelectrically convert incident light, a floatingdiffusion section configured to generate a voltage in accordance with anamount of charge carriers obtained by the photoelectric conversion, anda transfer section configured to transfer the charge carriers from thefirst photoelectric conversion section to the floating diffusionsection, and a second pixel configured to detect incidence of light.Readout of a signal is performed intermittently from each of theplurality of first pixels in accordance with the voltage, and an outputof the second pixel is monitored continuously.

The imaging element may further include an event detection sectionconfigured to output a signal synchronized with incidence of light onthe second pixel, on a basis of the output of the second pixel.

The imaging element may further include a driving section configured tocontrol readout of a signal from the first pixel in accordance with anoutput of a signal from the event detection section.

The driving section may control the transfer section in accordance withan output of a signal from the event detection section to cause thecharge carriers obtained by the photoelectric conversion to betransferred to the floating diffusion section, and control readout of asignal from the first pixel.

The imaging element may further include a detection section configuredto generate a digital signal indicating an amount of light incident onthe first pixel, on a basis of a signal read out from the first pixel.

The imaging element may, further include an output section configured tocalculate an amount of light incident on the pixel array section, on abasis of the digital signal obtained for each of the plurality of firstpixels.

The first pixel may further include a first amplification sectionconfigured to output a signal in accordance with the voltage generatedby the floating diffusion section, and a selection section configured toenter a conduction state or a non-conduction state in accordance withcontrol and, on entering the conduction state, output a signal outputtedfrom the first amplification section, to the detection section.

The second pixel may include a second photoelectric conversion sectionconfigured to photoelectrically convert incident light, and a secondamplification section configured to output a signal in accordance withcharge carriers obtained by the photoelectric conversion by the secondphotoelectric conversion section.

The first pixel may be a non-multiplication-type pixel, and the secondpixel may be a multiplication-type pixel.

A light receiving surface of the second pixel may be larger than a lightreceiving surface of the first pixel.

According to the first aspect of the present technology, there isprovided a driving method for driving an imaging element, the imagingelement including a pixel array section including a plurality of firstpixels each including a photoelectric conversion section configured tophotoelectrically convert incident light, a floating diffusion sectionconfigured to generate a voltage in accordance with an amount of chargecarriers obtained by the photoelectric conversion, and a transfersection configured to transfer the charge carriers from thephotoelectric conversion section to the floating diffusion section, anda second pixel configured to detect incidence of light, the drivingmethod including: a step of continuously monitoring an output of thesecond pixel and detecting incidence of light on the second pixel on abasis of the output of the second pixel; and a step of resetting thefirst pixel periodically, and controlling readout of a signal accordingto the voltage from the first pixel, in accordance with detection ofincidence of light on the second pixel.

According to the first aspect of the present technology, in the imagingelement including a pixel array section including a plurality of firstpixels each including a photoelectric conversion section configured tophotoelectrically convert incident light, a floating diffusion sectionconfigured to generate a voltage in accordance with an amount of chargecarriers obtained by the photoelectric conversion, and a transfersection configured to transfer the charge carriers from thephotoelectric conversion section to the floating diffusion section, anda second pixel configured to detect incidence of light, an output of thesecond pixel is continuously monitored and incidence of light on thesecond pixel is detected on a basis of the output of the second pixel,and the first pixel is reset periodically, and readout of a signalaccording to the voltage from the first pixel is controlled inaccordance with detection of incidence of light on the second pixel.

According to a second aspect of the present technology, an electronicdevice includes: a pixel array section including a plurality of firstpixels each including a photoelectric conversion section configured tophotoelectrically convert incident light, a floating diffusion sectionconfigured to generate a voltage in accordance with an amount of chargecarriers obtained by the photoelectric conversion, and a transfersection configured to transfer the charge carriers from thephotoelectric conversion section to the floating diffusion section, asecond pixel configured to detect incidence of light. Readout of asignal is performed intermittently from each of the plurality of firstpixels in accordance with the voltage, and an output of the second pixelis monitored continuously.

According to the second aspect of the present technology, an electronicdevice is provided with: a pixel array section including a plurality offirst pixels each including a photoelectric conversion sectionconfigured to photoelectrically convert incident light, a floatingdiffusion section configured to generate a voltage in accordance with anamount of charge carriers obtained by the photoelectric conversion, anda transfer section configured to transfer the charge carriers from thephotoelectric conversion section to the floating diffusion section, asecond pixel configured to detect incidence of light. Readout of asignal is performed intermittently from each of the plurality of firstpixels in accordance with the voltage, and an output of the second pixelis monitored continuously.

Advantageous Effects of Invention

According to a first aspect and a second aspect of the presenttechnology, stable driving is possible with low voltage and low powerconsumption, and furthermore a time resolution of detection can beensured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a configurational example of a radiationcounting apparatus.

FIG. 2 is a diagram showing a configurational example of a radiationcounting apparatus.

FIG. 3 is a diagram describing pixel outputs.

FIG. 4 is a diagram describing an example of operation of a radiationcounting apparatus.

FIG. 5 is a diagram showing a configurational example of a pixel formeasuring the amount of light.

FIG. 6 is a diagram showing an example of driving of a pixel duringradiation counting.

FIG. 7 is a diagram showing an example of driving of a pixel duringradiation counting.

FIG. 8 is a diagram showing an example of driving of a pixel duringradiation counting.

FIG. 9 is a diagram showing a configurational example of a pixeldesigned specially for light emission event detection.

FIG. 10 is a diagram showing a configurational example of a comparator.

FIG. 11 is a diagram showing a layout example of pixels.

FIG. 12 is a diagram showing an example of driving of a pixel and acomparator during radiation counting.

FIG. 13 is a diagram showing another configurational example of a pixeldesigned specially for light emission event detection.

FIG. 14 is a diagram showing another configurational example of a pixeldesigned specially for light emission event detection.

FIG. 15 is a diagram showing a configurational example of a lightdetector.

FIG. 16 is a diagram showing another configurational example of aradiation counting apparatus.

FIG. 17 is a diagram showing another configurational example of a lightdetector.

FIG. 18 is a diagram describing an example in which the presenttechnology is applied to flow cytometry.

FIG. 19 is a diagram describing event detection, light exposure, andreadout.

FIG. 20 is a diagram showing a configurational example of a lightdetector.

MODE(S) FOR CARRYING OUT THE INVENTION

Hereinbelow, embodiments to which the present technology is applied aredescribed with reference to the drawings.

First Embodiment <Configurational Example of Radiation CountingApparatus>

The present technology makes it possible to detect, with high accuracy,the amount and the incidence timing of feeble pulse light of radiationcounting or the like. In particular, according to the presenttechnology, when detecting the amount and the incidence timing of feeblepulse light, stable driving is possible with low voltage and low powerconsumption, and furthermore a sufficient time resolution can beensured.

The present technology can be applied to, for example, variouselectronic devices such as radiation counters and nuclear medicaldiagnosis apparatuses of flow cytometry, PET, etc. In the following,first, a description is given using as an example a case where thepresent technology is applied to a radiation counter.

A radiation counter to which the present technology is applied canperform radiation counting accurately, is small in size and light inweight, is resistant to environmental variations, and can be stablydriven with low voltage and low power consumption. Furthermore, theradiation counter can also ensure a time resolution of detectionsufficiently, and can be used also for coincidence counting of a nuclearmedical diagnosis apparatus such as a PET.

FIG. 1 and FIG. 2 are diagrams showing a configurational example of anembodiment of a radiation counting apparatus that is a radiation counterto which the present technology is applied. Note that FIG. 1 shows across-sectional view of a radiation counting apparatus 11, and FIG. 2shows a perspective view of the radiation counting apparatus 11.Further, portions corresponding between FIG. 1 and FIG. 2 are markedwith the same reference numerals.

The radiation counting apparatus 11 includes a light receiving section21 and a data processing section 22.

The light receiving section 21 includes a scintillator 31, a separatingwall 32, and a light detector 33. Note that, in FIG. 2, the illustrationof the separating wall 32 is omitted.

The scintillator 31 generates photons if radiation is incident. Thescintillator 31 contains, for example, Lu₂SiO₅:Ce (LSO), and isfashioned in a columnar shape of a 2-millimeter (mm) square.

The separating wall 32 covers the scintillator 31 and blocks visiblelight. However, in the separating wall 32, only a surface facing thelight detector 33 is opened. Further, the separating wall 32 preferablycontains, for example, a reflective substance that reflects light, suchas aluminum. Thereby, most of the photons generated in the scintillator31 can be caused to be incident on the light detector 33.

The light detector 33 includes, for example, a solid-state imagingelement such as a complementary metal oxide semiconductor (CMOS) imagesensor, and detects light incident from the scintillator 31 andgenerates a digital signal.

The light detector 33 has a light receiving surface facing thescintillator 31, and a plurality of (e.g., 100×100) pixel circuits 41are provided in a two-dimensional lattice configuration on the lightreceiving surface. Furthermore, on the light receiving surface of thelight detector 33, a pixel circuit 42 having a use at least differentfrom the use of the pixel circuit 41 is inserted in an array of one row.The pixel circuit 41 and the pixel circuit 42 are designed as, forexample, a non-multiplication-type pixel.

In the example shown in FIG. 2, the horizontal direction and thevertical direction are taken as the x direction and the y direction inFIG. 2, respectively, and a plurality of pixel circuits are provided tobe arranged in the x direction and the y direction on the lightreceiving surface. Note that herein only some of the plurality of pixelcircuits are marled with reference numerals; among the pixel circuitsshown by the quadrangles on the light receiving surface, the pixelcircuit marked with oblique lines shows the pixel circuit 42, and thepixel circuit not marked with oblique lines shows the pixel circuit 41.

Thus, in this example, it can be seen that one pixel row including pixelcircuits 42 that are arranged in the x direction is inserted in thearray of pixel circuits. Note that, hereinafter, the pixel circuit 41may be referred to as simply a pixel 41, and the pixel circuit 42 may bereferred to as simply a pixel 42.

The light detector 33 supplies a digital signal that is obtained by thepixel 41 or the pixel 42 receiving and photoelectrically convertinglight incident from the scintillator 31, to the data processing section22 via a signal line 43.

The data processing section 22 processes a digital signal supplied fromthe light detector 33 to perform radiation counting. Further, the dataprocessing section 22 records a time stamp of radiation that has enteredthe scintillator 31, and investigates the amount of light of a lightemission pulse generated in accordance with the entry of radiation intothe scintillator 31 and assesses the energy of radiation.

Here, the time stamp indicates the time of entry of radiation into thescintillator 31, and is generated on the basis of an output from thepixel 42. Further, the derivation of the amount of light of a lightemission pulse is performed on the basis of an output from the pixel 41.

Note that the method for inserting pixels 42, that is, the arrangementof pixels 42 on the light receiving surface of the light detector 33 maybe any type; for example, a plurality of rows of pixels 42 may beinserted. However, in order that pixels 41 used for measuring the amountof light can receive a large amount of light, the occupation area ofpixels 42 in the entire light receiving surface of the light detector 33may be set to less than or equal to a half, preferably less than orequal to ¼, of the light receiving surface.

Further, the scintillator 31 and the light detector 33 are preferablybonded together by an optical adhesive having an appropriate refractiveindex. Further, a light guide using fiberglass or the like may beinserted between the scintillator 31 and the light detector 33.

Furthermore, a radiation counter having a space resolution of a PET, agamma camera, or the like can be produced by tiling (arraying) a set ofthe scintillator 31 and the light detector 33 in the x direction and they direction in FIG. 2.

Meanwhile, in radiation counting, as shown in FIG. 1, a feeble lightemission pulse containing, for example, several thousand photons L12that are generated by, for example, one radiation carrier L11 beingphotoelectrically absorbed in the scintillator 31 is measured. In thelight detector 33, photons L12 of a light emission pulse generated inthis way are received by a pixel array including a plurality of pixels41 and a plurality of pixels 42, and each pixel performs an independentoutput in accordance with the light receiving condition.

For example, the pixel 41 accumulates, in this pixel 41, charge carriersgenerated by a photoelectric conversion element provided therein. Thepixel 41 can accumulate a plurality of charge carriers corresponding tothe incidence of a plurality of photons. That is, the output of thepixel 41 changes in accordance with the number of photons incident onthis pixel 41.

A signal that is an output of the pixel 41 and is in accordance with theaccumulated charge carriers of the pixel 41 is read out at a desiredtiming, as necessary, and is converted by an A/D converter to a digitalvalue having a gradation larger than 1 bit. Further, the pixel 41 has afunction of resetting the inside to a dark state where there is nophoton incidence.

On the other hand, the pixel 42 outputs a signal synchronized with areceived light emission pulse, in accordance with charge carriersgenerated by a photoelectric conversion element provided therein. Theoutput of each pixel 42 is monitored constantly during the period ofdetection of a light emission pulse, and is sensed as a light emissionevent of a light emission pulse.

Here, an example of outputs of a group of pixels 41 is shown in FIG. 3.

In FIG. 3, each quadrangle represents one pixel 41, and the numericalvalue in each of those quadrangles represents the value (digital value)of the output signal of the pixel 41.

In this example, if one least significant bit (LSB) is taken as theminimum unit, the value of an output signal on the occasion when thepixel 41 has received one photon corresponds to 10 LSBs. Each outputsignal includes a signal corresponding to the received photons andreadout noise, and there is a case where the value of the output signalis a negative value, depending on the magnitude of readout noise.

Note that, although herein the value of the output signal is written asa negative value as it is, the output signals of all the pixels 41 maybe processed by offsetting, or an output signal with a negative valuemay undergo processing of rounding the value up to zero.

Thus, the light detector 33 is an aggregate of pixels 41 each of whichis a high-sensitivity light detection cell having a gradation output.

The pixel 41 is a non-multiplication-type pixel that does not performelectron multiplication with a strong electric field like in an APD oran SiPM, and its output signal is minute. Hence, significant readoutnoise is contained in the output signal of the pixel 41, and thereforethe number of incident photons in the individual pixel 41 is ambiguous.However, the amount of light of a light emission pulse corresponding toone radiation carrier can be found with high accuracy by synthesizingthe output signals of the pixels 41.

For example, a photomultiplier or a common APD detects a light emissionpulse with a single detector, and generates an analog pulse inaccordance with the amount of light. On the other hand, an SiPM receivesa light emission pulse with a pixel array, but only a pixel that hasexperienced photon incidence outputs a certain charge pulse in a binarymanner. Then, the definitive output intensity is determined by thenumber of pulse-fired pixels.

That is, all of these pixels are different from the form of the lightdetector 33. Further, each of these light detectors including aphotomultiplier, an APD, or an SiPM has a response close to the responseof the pixel 42 in that each of these light detectors outputs a signalin synchronization with the incidence of photons as a light emissionpulse, and is clearly different from the pixel 41. Note that detailedconfigurational examples of the pixel 41 and the pixel 42 are describedlater.

<With Regard to Operation of Radiation Counting Apparatus>

Next, an example of the operation of the radiation counting apparatus 11is described.

It is assumed that, for example as shown in FIG. 4, a control circuit 71is obtained from a part of the light detector 33 and a part of the dataprocessing section 22, and the operation of the light detector 33 duringradiation counting is controlled by the control circuit 71. Note that,in FIG. 4, portions corresponding to those in the case of FIG. 1 aremarked with the same reference numerals, and a description thereof isomitted as appropriate. Further, in FIG. 4, the horizontal directionrepresents time.

Each pixel 41 receives a reset signal derived from the control circuit71, and the pixels 41 are reset on a periodic basis, preferably all atonce.

In FIG. 4, the dotted arrow connecting the control circuit 71 and thepixel 41 represents a reset signal, and the period between a round ofresetting and the next round of resetting of the pixel 41 by resetsignals is taken as a unit detection period. Each pixel 41 accumulates,during each unit detection period, charge carriers obtained byphotoelectrically converting photons incident from the scintillator 31,and releases the accumulated charge carriers in accordance with a resetsignal to reset this pixel 41.

Further, if radiation enters the scintillator 31 to generate a lightemission pulse and the pixel 42 of the light detector 33 receives thelight emission pulse, that is, photons of the light emission pulse, thelight detector 33 outputs an event signal in accordance with an outputsignal derived from the pixel 42.

The event signal is outputted when a light emission pulse is received bythe pixel 42, and is a signal indicating that one radiation carrier hasentered the scintillator 31.

In particular, in the light detector 33, the output of the pixel 42 ismonitored continuously, that is, constantly to detect the incidence of alight emission pulse on the pixel 42; if the incidence of a lightemission pulse on the pixel 42 is detected, an event signal indicatingthat fact is outputted immediately. In other words, the event signal canbe said to be a signal that is outputted substantially simultaneouslywith the generation of a light emission pulse, that is, a signalsynchronized with the incidence of a light emission pulse on the pixel42. In the following, the generation of a light emission pulse indicatedby an event signal may be referred to as a light emission event.

The control circuit 71 supplied with an event signal derived from thelight detector 33 generates a data object DB11 and records the timestamp of the light emission event on the data object DB11, and causesthe readout of an output signal from the pixel 41 to start.

In this example, the readout of an output signal from the pixel 41 hasbeen started since the timing indicated by arrow A11, and output signalsare read out sequentially from the plurality of pixels 41 arranged onthe light receiving surface.

Then, for example, the control circuit 71 records, on the data objectDB11, the sum total (additional value) of the values of output signalsread out from the pixels 41, as a value indicating the amount of lightof the light emission pulse. In the following, the sum total of thevalues of output signals of all the pixels 41 may be referred to as anoutput synthetic value.

Thus, while the output of the pixel 42 is monitored constantly, thereadout of a signal from the pixel 41 is performed only when a lightemission event is detected. That is, the readout of a signal from thepixel 41 is performed intermittently.

If the readout of output signals from all the pixels 41 is completed,then the pixel 41 is reset again at a prescribed timing, and the nextunit detection period is started. The data object DB11 in which a timestamp and an output synthetic value of the pixels 41 make a set in thisway is saved in an external memory or a memory in the control circuit71.

Note that the whole or a part of the function of the control circuit 71may be enclosed in the light detector 33, or may be enclosed in the dataprocessing section 22.

Further, for example, in a case where the radiation source is known, thecontrol circuit 71 may further assess whether the output synthetic valueis in a proper energy range or not, and may discard the data object ofan output synthetic value that is assessed as not being in a properenergy range, that is, regarded as an error. Furthermore, after theassessment of whether the output synthetic value is in a proper energyrange or not, the control circuit 71 may save only the time stamp andsome flags. Conversely, in a case where the time stamp is unnecessary,the control circuit 71 may save only data related to the outputsynthetic value.

The detection of a light emission pulse by an operation like the aboveenables a great reduction in the amount of data processing and powerconsumption, because readout is performed and the subsequent processingand recording are performed only on a significant light emission signal,that is, an output signal obtained by receiving a light emission pulse.

Thus, the light detector 33 can be used for all types of pulse lightdetection, but is particularly effective in a case where the timing ofgeneration of pulse light is unknown. For example, radiation countingsuch as survey meters, PET, and single photon emission computedtomography (SPECT), fluorescence and side scattered light detection inflow cytometry, and the like fall under such a case.

Further, the present technology is effective in fields in which pulsedetection requires a nanosecond-level time resolution, particularly suchas coincidence counting of gamma rays in PET.

The pixel 41, which is of a charge carrier accumulation type and theoutput of which does not synchronize with a light emission pulse, isadvantageous for downsizing, reduction of voltage, environmentaltolerance, and stable operation, but has difficulty in obtaining ananosecond-level time resolution by itself. This is because the pixel 41by itself cannot find what time point a light emission event hasoccurred at, and cannot obtain a time resolution exceeding a frame ratethat is obtained when the readout time of output signals of one rowincluding pixels 41 or one round of readout of signals from all thepixels 41 is taken as one frame.

Thus, by additionally providing the pixel 42 designed specially forlight emission event detection in the light detector 33, the radiationcounting apparatus 11 makes it possible to generate a correct time stampand achieve both the ensuring of a high time resolution and stabledriving with a reduced size, low voltage, and low power consumption.

<Configurational Example of Pixel for Measuring Amount of Light>

Next, a more detailed configuration of the pixel 41 for measuring theamount of light of a light emission pulse is described.

The pixel 41 for measuring the amount of light of a light emission pulsehas the configuration shown in FIG. 5, for example.

That is, the pixel 41 includes a photodiode 101, an accumulation node102, a transfer transistor 103, a detection node 104, a reset transistor105, an amplification transistor 106, and a selection transistor 107.

For example, an n-type metal-oxide-semiconductor (MOS) transistor isused as the transfer transistor 103, the reset transistor 105, theamplification transistor 106, and the selection transistor 107. Further,the photodiode 101 is designed as a non-multiplication-type photodiode.

The photodiode 101 is connected to the transfer transistor 103 via theaccumulation node 102, and receives and photoelectrically convertsphotons of a light emission pulse incident from the scintillator 31.That is, the photodiode 101 generates pairs of electrons and holes fromphotons incident on a silicon substrate of the pixel 41 by photoelectricconversion, and accumulations electrons (charge carriers) of the pairsin the accumulation node 102. The photodiode 101 is preferably of anembedded type that is completely depleted during charge carrier releaseby resetting.

The transfer transistor 103 transfers charge carriers from theaccumulation node 102 to the detection node 104, in accordance withcontrol of a row driving circuit 111. The detection node 104 includes,for example, a floating diffusion layer; and accumulates charge carrierstransferred from the transfer transistor 103, and generates an analogvoltage in accordance with the amount of accumulated charge carriers.This voltage is applied to the gate of the amplification transistor 106.

The reset transistor 105 extracts the charge carriers accumulated in theaccumulation node 102 and the detection node 104 to a power source 112,that is, causes the accumulated charge carriers to be released, andperforms initialization. The gate of the reset transistor 105 isconnected to the row driving circuit 111, the drain is connected to thepower source 112, and the source is connected to the detection node 104.

The row driving circuit 111 controls the reset transistor 105 to the ONstate simultaneously with the transfer transistor 103, for example; andthereby causes the electrons (charge carriers) accumulated in theaccumulation node 102 to be released to the power source 112, andinitializes the pixel 41 to a dark state prior to charge carrieraccumulation, that is, a state where light is not incident on thephotodiode 101 yet. In other words, the photodiode 101 is reset.Further, the row driving circuit 111 controls only the reset transistor105 to the ON state; and thereby causes the charge carriers accumulatedin the detection node 104 to be released to the power source 112, andinitializes the amount of charge carriers.

The amplification transistor 106 amplifies the voltage of its gate. Thegate of the amplification transistor 106 is connected to the detectionnode 104, the drain is connected to the power source 112, and the sourceis connected to the selection transistor 107. The amplificationtransistor 106 and a constant current circuit 113 form a sourcefollower, and the voltage of the detection node 104 is outputted by theamplification transistor 106 to a vertical signal line 114, with a gainof just under 1.

An electrical signal of the voltage outputted from the amplificationtransistor 106 to the vertical signal line 114 via the selectiontransistor 107 is a signal indicating the amount of light of photonsincident on the pixel 41, and is acquired by a detection circuit 115including a not-illustrated A/D conversion circuit.

The selection transistor 107 enters the conduction state or thenon-conduction state in accordance with control of the row drivingcircuit 111, and during the conduction state, outputs an electricalsignal that is outputted from the amplification transistor 106 and is inaccordance with the voltage of the detection node 104. The gate of theselection transistor 107 is connected to the row driving circuit 111,the drain is connected to the amplification transistor 106, and thesource is connected to the vertical signal line 114.

During the period from when the photodiode 101 is reset to when thereadout of a signal in accordance with the amount of light of incidentphotons is performed, the pixel 41 accumulates charge carriers obtainedby photoelectric conversion in the inside; and during readout, the pixel41 outputs a signal in accordance with the accumulated charge carriers.In the pixel 41, such accumulation and readout of charge carriers in aunit period are performed repeatedly; if photons of a light emissionpulse are incident on the pixel 41 during charge carrier accumulation,the result can be obtained at the time of readout.

Meanwhile, a feature of such a photodiode 101 of an embedded type isthat the detection node 104 and the accumulation node 102 of thephotodiode 101 are not capacitively coupled during readout. As a result,as the parasitic capacitance of the detection node 104 is reduced, theefficiency of conversion of accumulated charge carriers to a voltagesignal is improved, and the sensitivity to the incidence of one photoncan be improved.

Further, since the detection node 104 and the accumulation node 102 arenot capacitively coupled during readout, the conversion efficiency doesnot worsen even if the photodiode 101 is expanded to a larger size, thatis, even if the area of the light receiving surface of the photodiode101 is increased.

Therefore, as the photodiode 101 is expanded to a larger size, a largeramount of photons can be received, and thus the sensitivity per pixel 41to the same luminous flux density is improved. Note that similarproperties are observed also in a MOS-type photoelectric conversionelement.

Further, such a pixel 41 generally does not involve electronmultiplication like in an APD, a SiPM, or a photomultiplier. Therefore,the output of such a pixel 41 is affected by readout noise derived fromthe amplification transistor 106 or a later-stage A/D conversioncircuit; however, the effect is relatively minimized by using theproperties mentioned above to maximize pixel sensitivity.

That is, the parasitic capacitance of the detection node 104 is made assmall as possible, and the photodiode 101 is expanded to as large a sizeas possible to the extent that single-electron transfer is possible;thereby, the signal-to-noise ratio (S/N ratio) of the output of thepixel 41 is maximized. Thus, a pixel 41 as a high-sensitivity detectorused in the radiation counting apparatus 11 is obtained.

<Example 1 of Driving of Pixel for Measuring Amount of Light>

Next, the driving of the pixel 41, that is, the operation of the pixel41 during radiation counting is described. FIG. 6 is a timing chartshowing an example of the driving of the pixel 41 during radiationcounting.

Note that, in FIG. 6, the horizontal direction represents time. Further,in FIG. 6, polygonal line DP11 to polygonal line DP13 represent the ONor OFF states of the transfer transistor 103, the reset transistor 105,and the selection transistor 107, respectively.

That is, the section where each of polygonal line DP11 to polygonal lineDP13 is convex upward indicates a section where each transistor is inthe ON state, that is, the conduction state; and the section where eachof polygonal line DP11 to polygonal line DP13 is convex downwardindicates a section where each transistor is in the OFF state, that is,the non-conduction state.

First, at timing T1 immediately before a light exposure period, the rowdriving circuit 111 controls both the transfer transistor 103 and thereset transistor 105 to the ON state. That is, the row driving circuit111 sets a driving signal to be supplied to the gates of thosetransistors to a high level (H-level), and thereby causes the transfertransistor 103 and the reset transistor 105 to be switched on.

By this control, all the charge carriers accumulated in the accumulationnode 102 between the photodiode 101 and the transfer transistor 103 arereleased to the power source 112, and the photodiode 101 is reset. Inthe following, such control may be referred to as photodiode (PD)resetting.

After the PD resetting, the row driving circuit 111 controls thetransfer transistor 103 to the OFF state. By this control, theaccumulation node 102 enters a floating state, and a new round of chargecarrier accumulation is started. That is, the PD resetting is removed,and a light exposure period of the pixel 41 is started.

In the light exposure period, the photodiode 101 receives andphotoelectrically converts light (photons) incident from thescintillator 31. Then, charge carriers (electrons) obtained byphotoelectric conversion by the photodiode 101 are continuouslyaccumulated in the accumulation node 102.

Further, after the PD resetting, more specifically, after the start ofthe light exposure period, the row driving circuit 111 controls thereset transistor 105 to the OFF state. Note that, during charge carrieraccumulation, the reset transistor 105 may remain in the ON state.

On the other hand, the selection transistor 107 is controlled to the OFFstate in order to allow access to another pixel 41 connected to thevertical signal line 114, that is, the readout of a signal from anotherpixel 41.

Next, the operation of readout of accumulation signals is described. Inthe readout operation, first the sampling of reset signals is performedas pre-processing, and next the light exposure period is ended and thesampling of accumulation signals is performed. That is, the readoutoperation is performed by two-step sampling.

Here, timing T2 is a timing at which a light emission event is detected,that is, the timing indicated by arrow A11 of FIG. 4, for example. Notethat, in a case where the readout of a signal is performed for each rowof pixels 41, the timing at which the row is specified as a readout rowafter the timing indicated by arrow A11 serves as timing T2.

Then, at timing T2 prior to the end of the light exposure period, therow driving circuit 111 controls the reset transistor 105 and theselection transistor 107 to the ON state.

By the control of setting the selection transistor 107 to the ON state,the pixel 41 enters a selection state, and the pixel 41 is electricallyconnected to the vertical signal line 114. That is, it becomes possiblefor an output from the amplification transistor 106 to be read by thedetection circuit 115 via the selection transistor 107 and the verticalsignal line 114.

Further, by the control of setting the reset transistor 105 to the ONstate, the detection node 104, which is the input of the amplificationtransistor 106, and the power source 112 are short-circuited. Thereby, areference electric potential is generated in the selected pixel 41. Thatis, the electric potential of the detection node 104 is reset to theelectric potential of the power source 112.

If a pulse period, that is, a period equivalent to one pulse has elapsedfrom timing T2, the row driving circuit 111 controls the resettransistor 105 to the OFF state. By this control, the electric potentialof the detection node 104 undergoes coupling with the gate of the resettransistor 105 and decreases from the reference electric potential tosome degree, and enters a floating state.

Furthermore, on this occasion, significant kT/C noise is generated inthe detection node 104. In general, a floating diffusion layer is usedas the detection node 104. Thus, in the following, the control that setsthe reset transistor 105 to the ON state at timing T2 and then sets thereset transistor 105 to the OFF state to reset the detection node 104may be referred to as FD resetting.

If FD resetting is performed, then the detection circuit 115 performsone or more times (for example, four times) of sampling during theperiod up to the end of the light exposure period.

Specifically, the detection circuit 115 A/D-converts a signal of theelectric potential of the vertical signal line 114, as a reset signal ofthe pixel 41, and obtains a digital signal Ds1. The detection circuit115 performs such sampling several times.

By such sampling, a signal in accordance with the voltage of thedetection node 104 is read out to the detection circuit 115 via theamplification transistor 106, the selection transistor 107, and thevertical signal line 114, and the read out signal is A/D-converted intoa digital signal Ds1.

The sampling of reset signals by the detection circuit 115 is treated asthe first round of readout in correlated double sampling.

Further, at timing T3 before the light exposure period ends, the rowdriving circuit 111 controls the transfer transistor 103 to the ONstate.

By this control, charge carriers accumulated in the accumulation node102 are transferred to the detection node 104 by the transfer transistor103. On this occasion, if the potential of the detection node 104 issufficiently deep, all the electrons (charge carriers) accumulated inthe accumulation node 102 are transferred to the detection node 104, andthe accumulation node 102 enters a complete depletion state.

If a pulse period has elapsed from timing T3, the row driving circuit111 controls the transfer transistor 103 to the OFF state.

By this control, the electric potential of the detection node 104 dropsby an amount equivalent to the amount of accumulated charge carriers ascompared to before the driving of the transfer transistor 103. That is,the potential of the detection node 104 becomes shallower. A voltageequivalent to this electric potential drop is amplified by theamplification transistor 106, and is outputted to the vertical signalline 114 via the selection transistor 107.

In other words, a signal in accordance with the voltage of the detectionnode 104 is supplied to the detection circuit 115 via the amplificationtransistor 106, the selection transistor 107, and the vertical signalline 114.

If the transfer transistor 103 is set to the OFF state in this way, thelight exposure period ends.

If the light exposure period ends, then the detection circuit 115performs one or more times (for example, four times) of sampling duringthe period up to timing T4.

Specifically, the detection circuit 115 A/D-converts a signal of theelectric potential of the vertical signal line 114, as an accumulationsignal of the pixel 41, and obtains a digital signal Ds2. The detectioncircuit 115 performs such sampling several times.

By such sampling, a signal in accordance with the voltage of thedetection node 104 is read out to the detection circuit 115 via theamplification transistor 106, the selection transistor 107, and thevertical signal line 114, and the read out signal is A/D converted intoa digital signal Ds2.

The sampling of accumulation signals by the detection circuit 115 istreated as the second round of readout in correlated double sampling.

The detection circuit 115 compares the sampled accumulation signal,namely the digital signal Ds2, and the reset signal, namely the digitalsignal Ds1, and assesses the amount of photons incident on the pixel 41,on the basis of the comparison result.

For example, the detection circuit 115 adds all the plurality of digitalsignals Ds1 obtained by sampling, and takes the addition result as asignal indicating a reset level of the pixel 41.

Similarly, the detection circuit 115 adds all the plurality of digitalsignals Ds2 obtained by sampling, and takes the addition result as asignal indicating a signal level of the pixel 41.

Then, the detection circuit 115 calculates the difference between theresult of addition (additional value) of the digital signals Ds1 and theresult of addition (additional value) of the digital signals Ds2, as anet accumulation signal, that is, the definitive digital output signalindicating the amount of light of photons incident on the pixel 41, andoutputs the result to a later stage.

Note that, although herein an example in which the additional values ofthe digital signals Ds1 and the digital signals Ds2 are found isdescribed, it is also possible to calculate the average values of thedigital signals Ds1 and the digital signals Ds2, and take the differencebetween the average values as the definitive output signal of the pixel41.

Further, at timing T4, the row driving circuit 111 controls theselection transistor 107 to the OFF state, and the readout of an outputsignal in one unit detection period from the pixel 41 ends.

In the driving of the pixel 41 like the above, kT/C noise generatedduring FD resetting is offset by taking the difference between thedigital signal Ds1 and the digital signal Ds2 as a net accumulationsignal, and an output signal with small noise can be obtained in theend. Further, since the signal is leveled by performing the sampling ofthe digital signal Ds1 and the digital signal Ds2 multiple times, anoutput signal with small noise can be obtained.

As described above, the light exposure period of each pixel 41 is theperiod between the PD resetting operation and the operation of readoutof the accumulation signal. More specifically, the period from when thetransfer transistor 103 becomes OFF immediately after timing T1 to whenthe transfer transistor 103 becomes OFF immediately after timing T3 istaken as the light exposure period.

If during the light exposure period photons are incident on thephotodiode 101 and charge carriers are generated, they are thedifference between the reset signal and the accumulation signal, and thevalue of the difference is derived as the value of an output signal bythe detection circuit 115, in accordance with the procedure describedabove.

Note that, in the example of the operation (sequence) shown in FIG. 4,the readout of an accumulation signal corresponding to accumulatedcharge carriers is not performed if a light emission event is notdetected. That is, the readout of an accumulation signal from the pixel41 is performed intermittently. Hence, in a case where a light emissionevent has not been detected, the period between a round of PD resettingand the next round of PD resetting serves as a light exposure period. Amain object of such periodic PD resetting in a dark state is the releaseof noise charge carriers accumulated due to dark current, etc.

<Example 2 of Driving of Pixel for Measuring Amount of Light>

The driving sequence of the pixel 41 described with reference to FIG. 6is a typical example of the driving sequence of an accumulation-typepixel; but different types of driving may be performed in accordancewith the use. That is, for example, the pixel 41 may be driven in themanner shown in FIG. 7.

Note that, in FIG. 7, the horizontal direction represents time. Further,in FIG. 7, polygonal line DP21 to polygonal line DP23 represent the ONor OFF states of the transfer transistor 103, the reset transistor 105,and the selection transistor 107, respectively. In particular, the statewhere each of polygonal line DP21 to polygonal line DP23 is convexupward indicates the ON state, and the state where each of polygonalline DP21 to polygonal line DP23 is convex downward indicates the OFFstate.

In the example shown in FIG. 7, the driving of the transfer transistor103 is the same driving as the example shown in FIG. 6. Further, in theexample shown in FIG. 7, also the driving of timing T11 is the same asthe driving of timing T1 shown in FIG. 6, and PD resetting is performedat timing T11.

If at timing T11 PD resetting is performed and the transfer transistor103 is switched off to start light exposure, the reset transistor 105 iscontrolled to the OFF state, and then the row driving circuit 111controls the selection transistor 107 to the ON state at timing T12.Then, during the period up to the end of the light exposure period, thedetection circuit 115 performs one or more times of sampling of a resetsignal.

In the example shown in FIG. 6, as pre-processing of the readoutoperation, FD resetting is performed immediately before the end of thelight exposure period, and then the sampling of reset signals isperformed; on the other hand, in the example shown in FIG. 7, only thesampling of reset signals is performed in advance immediately after thestart of the light exposure period. Further, on this occasion, FDresetting has been performed in association with the PD resetting.

The result of the sampling of reset signals performed after timing T11,that is, a digital signal Ds1 obtained in each round of sampling istemporarily held by the detection circuit 115.

If the sampling of reset signals ends, then the row driving circuit 111controls the selection transistor 107 to the OFF state.

Further, if a light emission event is detected by the reception of alight emission pulse by the pixel 42, at timing T13 the row drivingcircuit 111 controls the transfer transistor 103 to the ON state tocause the transfer of charge carriers from the accumulation node 102 tothe detection node 104 to start.

Then, after that, the row driving circuit 111 controls the transfertransistor 103 to the OFF state to cause the transfer of charge carriersto end; thereby, the light exposure period ends.

At timing T14, the row driving circuit 111 controls the selectiontransistor 107 to the ON state. Thereby, a signal in accordance with thevoltage of the detection node 104 is supplied to the detection circuit115 via the amplification transistor 106, the selection transistor 107,and the vertical signal line 114; thus, the detection circuit 115performs one or more times of sampling of an accumulation signal. As aresult, a digital signal Ds2 is obtained.

If the sampling of accumulation signals is performed, then the rowdriving circuit 111 controls the selection transistor 107 to the OFFstate, and the readout of an output signal in one unit detection periodfrom the pixel 41 ends.

An advantage of the driving shown in FIG. 7 in the above manner is thattiming T11 and timing T13 of light exposure, and timing T12 and timingT14 of the sampling of signals are independent, and flexible timingsetting becomes possible.

For example, also in a case where a plurality of pixels 41 are connectedto one detection circuit 115, it is possible to introduce what is calleda global shutter, in which the start and end of light exposure aresimultaneously performed in all the pixels 41 all at once, and thereadout of signals is sequentially performed on a pixel 41 basis.

Further, in the driving described with reference to FIG. 6, if the PDresetting at timing T1 is omitted and charge carrier release from thephotodiode 101 associated with the charge carrier transfer at timing T3at the time of readout is used also as PD resetting, the next lightexposure period is started from immediately after this charge carrierrelease.

<Example 3 of Driving of Pixel for Measuring Amount of Light>

Further, in the driving described with reference to FIG. 7, pulseapplication to the transfer transistor 103, that is, the ON/OFF controlof the transfer transistor 103 at timing T11 may be omitted, and onlythe detection node 104 may be reset. Thereby, a dead period in whichphotons incident on the pixel 41 are not sensed becomes almost zero.

In such a case, the driving of the pixel 41 is as shown in FIG. 8. Notethat, in FIG. 8, portions corresponding to those in the case of FIG. 7are marked with the same reference numerals, and a description thereofis omitted as appropriate. Further, in FIG. 8, polygonal line DP31represents the ON or OFF state of the transfer transistor 103.

In the example shown in FIG. 8, at timing T11 the transfer transistor103 is not driven but kept in the OFF state, and only the resettransistor 105 is set to the ON state and then to the OFF state; thus,FD resetting is performed. Thereby, charge carriers accumulated in thephotodiode 101 and the accumulation node 102 are held as they are, andonly the detection node 104 is reset.

Then, at timing T12 the selection transistor 107 is controlled to the ONstate, then the sampling of reset signals is performed, and after thatthe selection transistor 107 is controlled to the OFF state.

Further, at timing T13 the transfer transistor 103 is controlled to theON state, and the transfer of charge carriers from the photodiode 101and the accumulation node 102 to the detection node 104 is started.Then, after that, if the transfer transistor 103 is controlled to theOFF state, the light exposure period ends.

On this occasion, the transfer of charge carriers from the photodiode101 and the accumulation node 102 to the detection node 104 serves as PDresetting at the same time, and the next round of charge carrieraccumulation in the photodiode 101 is started. That is, at the same timeas when the transfer transistor 103 is set to the OFF state and thelight exposure period ends, the next light exposure period is started.

Therefore, in the driving shown in FIG. 8, the dead period of detectionof the amount of light of a light emission pulse by the pixel 41 iszero. Further, the time of accumulation of charge carriers in thephotodiode 101 (the light exposure period) is equal to the time of oneunit detection period, that is, a time equivalent to one frame.

Further, the subsequent driving at and after timing T14 is similardriving to the example shown in FIG. 7.

<Configurational Example of Pixel Designed Specially for Light EmissionEvent Detection>

Next, a more detailed configuration of the pixel 42 for detecting thelight emission of a light emission pulse is described.

The pixel 42 designed specially for light emission event detection todetect the generation of a light emission pulse, that is, a lightemission event has the configuration shown in FIG. 9, for example.

That is, the pixel 42 includes a photodiode 141, an accumulation node142, a transfer transistor 143, a detection node 144, a reset transistor145, an amplification transistor 146, and a reset transistor 147.

In this example, in order to suppress the increase in the number ofmanufacturing processes as much as possible, a structure similar to thepixel 41 is employed as the structure of the pixel 42.

That is, the transfer transistor 143, the detection node 144, theamplification transistor 146, and the reset transistor 145 provided inthe pixel 42 have the same size and the same structure as the transfertransistor 103, the detection node 104, the amplification transistor106, and the reset transistor 105 of the pixel 41, respectively.Further, the photodiode 141 is designed as a non-multiplication-typephotodiode.

The photodiode 141 receives and photoelectrically converts photons of alight emission pulse incident from the scintillator 31, and accumulatesthe resulting charge carriers (electrons) in the accumulation node 142.

In the pixel 42, the transfer transistor 143 and the reset transistor147 are connected to the photodiode 141 via the accumulation node 142,and charge carriers accumulated in the accumulation node 142 aretransferred to the detection node 144 by the transfer transistor 143.

The detection node 144 includes, for example, a floating diffusionlayer; and accumulates charge carriers transferred from the transfertransistor 143, and generates an analog voltage in accordance with theamount of accumulated charge carriers.

The drain of the reset transistor 145 is connected to a power source151, and the source is connected to the detection node 144. Further, thedrain of the reset transistor 147 is connected to the power source 151,and the source is connected to the accumulation node 142. The resettransistor 145 and the reset transistor 147 are driven by, for example,the row driving circuit 111.

Further, the gate of the amplification transistor 146 is connected tothe detection node 144, the drain is connected to the power source 151,and the source is connected to a vertical signal line 153. In the pixel42, the amplification transistor 146 serves as a source followertogether with a constant current circuit 152; and the charge (voltage)of the detection node 144 is amplified by the amplification transistor146, and is outputted as a voltage signal to the vertical signal line153.

However, the pixel 42 is designed specially for light emission eventdetection; hence, in the radiation counting apparatus 11, constantmonitoring of the output of the pixel 42, that is, the vertical signalline 153 is necessary.

Thus, the transfer transistor 143 is constantly kept in the ON state;and charge carriers obtained by photoelectric conversion in thephotodiode 141 are immediately transferred to the detection node 144,and are reflected in the output of the pixel 42.

Further, in the radiation counting apparatus 11, the vertical signallines 153 of a plurality of pixels 42 are short-circuited, and thereby1/f noise and thermal noise derived from the amplification transistors146 are leveled and reduced.

Further, during the period of detection of a light emission event, thevertical signal line 153 is electrically connected to a comparator 154all the time. Then, a signal outputted from the pixel 42 to the verticalsignal line 153 and a comparative electric potential CV are compared bythe comparator 154, and the comparison result is outputted as an eventsignal via an output amplifier 155.

Further, the comparator 154 has the configuration shown in FIG. 10, forexample.

The comparator 154 shown in FIG. 10 is a circuit of a common comparator;the electric potential of an input terminal 181 is set to thecomparative electric potential CV, and the vertical signal line 153 isconnected to an input terminal 182. Further, an output terminal 183 isconnected to the output amplifier 155.

In such a comparator 154, auto-zero is performed by switching on thegate of a PMOS connected to a driving signal line 184; if auto-zero isperformed, the output of the comparator 154 becomes an intermediateelectric potential between upper and lower reference values with whichcomparison assessment is performed.

<Layout Example of Pixels in Light Receiving Surface>

Further, the photodiode 141 provided in the pixel 42 is expanded to, forexample, twice the size of the photodiode 101 provided in the pixel 41.Accordingly, also the size of the pixel 42 is twice the size of thepixel 41, and pixels 42 are provided in the row direction with twice thepitch of pixels 41, for example.

This is in order to enhance the average amount of the signal per pixelwith respect to a single round of pulse light emission, that is, thevalue of the output signal of the pixel 42.

In light emission event detection, the average value of the outputsignal of each pixel 42 and the comparative electric potential CV arecompared by the comparator 154, and whether a light emission event, thatis, the incidence of radiation has occurred or not is detected on thebasis of the comparison result. Hence, the area of the light receivingportion of the pixel 42, that is, the light receiving surface of thephotodiode 141 is set larger than the light receiving surface of thephotodiode 101 of the pixel 41 so that, in a case where a light emissionevent has occurred, the value of each output signal is as large aspossible.

In such a case, charge carriers unable to be transferred from theaccumulation node 142 to the detection node 144 may be generated, butthe reset transistor 147 is provided in the pixel 42 additionally sothat those charge carriers are completely released by resetting.

Here, a layout example of the pixel 41 and the pixel 42 on the lightreceiving surface of the light detector 33 is shown in FIG. 11. Notethat, in FIG. 11, portions corresponding to those in the case of FIG. 5or FIG. 9 are marked with the same reference numerals, and a descriptionthereof is omitted as appropriate. Further, in FIG. 11, the horizontaldirection represents the row direction, that is, the x direction in FIG.2, and the vertical direction represents the column direction, that is,the y direction in FIG. 2.

In the example of FIG. 11, pixels 42 are provided in the row directionwith twice the pitch of pixels 41, and the size of the photodiode 141 ofthe pixel 42 is a little more than twice the size of the photodiode 101of the pixel 41.

Accordingly, in the pixel 42, a second reset transistor 147 is connectedto the photodiode 141 in addition to the reset transistor 145 that issimilar to the reset transistor of the pixel 41. Thereby, duringresetting, the charge carriers of the whole photodiode 141 are releasedwith reliability.

<Example of Driving of Pixel Designed Specially for Light Emission EventDetection>

Next, the driving of the pixel 42 and the comparator 154, that is, theoperation of the pixel 42 and the comparator 154 during radiationcounting is described. FIG. 12 is a timing chart showing an example ofthe driving of the pixel 42 and the comparator 154 during radiationcounting.

Note that, in FIG. 12, the horizontal direction represents time.Further, in FIG. 12, polygonal line DP41 represents the ON or OFF statesof the reset transistor 145 and the reset transistor 147. For polygonalline DP41, the state of being convex upward indicates being in the ONstate, and the state of being convex downward indicates being in the OFFstate.

Further, curved line DP42 represents the electric potential of thevertical signal line 153, and polygonal line DP43 represents theelectric potential of the driving signal line 184, that is, the voltagelevel of a driving signal that is supplied to the comparator 154 by thedriving signal line 184. Further, polygonal line DP44 represents thecomparative electric potential CV, and curved line DP45 represents theelectric potential (level) of a signal indicating the comparison resultoutputted from the comparator 154.

In the example of FIG. 12, the pixel 42 is reset periodically along withthe pixel 41, and dark current accumulated between those rounds ofresetting is released. That is, the operating process of the pixel 42and the comparator 154 includes a resetting operation (resettingsequence) and the output of an event signal upon receiving a lightemission pulse.

In the resetting operation, at timing T21, the row driving circuit 111controls the reset transistor 145 and the reset transistor 147 providedin the pixel 42 to the ON state. Thereby, all the charge carriers of thephotodiode 141 are released, and the electric potential of the detectionnode 144 is reset to the voltage of the power source 151. That is, theresetting of the photodiode 141 and the detection node 144 is performed.Here, the timing T21 of resetting is set to, for example, the same timeas the timing T1 of resetting of the pixel 41 shown in FIG. 6.

Further, after timing T21, the row driving circuit 111 controls thereset transistor 145 and the reset transistor 147 to the OFF state, andremoves the resetting of the photodiode 141 and the detection node 144.

If the reset transistor 145 is switched off, the detection node 144undergoes coupling with the gate of the reset transistor 145 andstabilizes at a level slightly reduced from the voltage supplied by thepower source 151, and the level is reflected in the vertical signal line153 almost linearly.

On the other hand, in the comparator 154, at timing T21, a drivingsignal to be supplied to the gate of the PMOS via the driving signalline 184 is set to a low level (L-level), and thereby the gate of thePMOS becomes ON and auto-zero operation is performed.

By this auto-zero operation, the input terminal 181 and the inputterminal 182 of the comparator 154 are short-circuited so that theelectric potential of the vertical signal line 153 and the comparativeelectric potential CV enter a balance state.

Consequently, a signal that is outputted from the output terminal 183 ofthe comparator 154 to the output amplifier 155 and that indicates thecomparison result in the comparator 154 has an intermediate electricpotential between upper and lower reference values with which comparisonassessment is performed. After that, a driving signal to be supplied tothe gate of the PMOS via the driving signal line 184 is set to anH-level, and the auto-zero operation is completed.

Further, after the auto-zero operation is completed, at timing T22, aprescribed negative offset is added to the comparative electricpotential CV, and the resetting of the pixel 42 and the comparator 154is completed.

On this occasion, if at timing T22 a negative offset is added to thecomparative electric potential CV, the output of the comparator 154,that is, a signal indicating the comparison result stabilizes at theH-level.

Note that the amount of the offset herein serves as a reference forassessing the presence or absence of light incidence, and needs to beappropriately set to such a level that false detection due to variationsin characteristics and noise of the comparator and the pixel 42 designedspecially for light emission event detection does not occur frequentlyand furthermore high sensitivity is obtained. To this end, a calibrationcircuit that cancels variations in characteristics or a mechanism thatprograms a level for each chip may be installed.

After that, if, for example at timing T23, photons of a light emissionpulse are incident on the pixel 42 from the scintillator 31, thephotoelectric conversion of the photons incident from the scintillator31 is performed in the photodiode 141 of the pixel 42. Charge carriersobtained by photoelectric conversion in the photodiode 141 areimmediately transferred to the detection node 144 via the accumulationnode 142 and the transfer transistor 143.

Consequently, the electric potential of the detection node 144 drops byan amount equivalent to the charge carriers transferred to the detectionnode 144, and a voltage equivalent to this drop is amplified by theamplification transistor 146, and is outputted to the vertical signalline 153; thus, the level of the vertical signal line 153 decreases.

Then, if the average value of signals outputted from a plurality ofpixels 42 that are short-circuited with vertical signal lines 153becomes smaller than the comparative electric potential CV, that is,exceeds the offset added to the comparative electric potential CV, asignal that is an output of the comparator 154 and indicates thecomparison result is reversed from the H-level to the L-level.

The comparison result is amplified by the output amplifier 155, and isoutputted as an event signal. That is, in synchronization with thegeneration of a light emission pulse in the scintillator 31, an eventsignal indicating the generation of the light emission pulse isoutputted from the output amplifier 155. Thus, in this example, an eventsignal that is outputted from the output amplifier 155 when thecomparison result is an L-level signal serves as an event signal thatindicates that a light emission event is detected.

<Another Configurational Example 1 of Pixel Designed Specially for LightEmission Event Detection>

Note that, in the above, an example in which a structure similar to thepixel 41 for measuring the amount of light is employed as the structureof the pixel 42 designed specially for light emission event detection inorder to suppress the increase in the number of manufacturing processesof the light detector 33 is described. However, it is preferable thatthe pixel 42 emit a clear output in synchronization with pulse light,and ensure a high S/N ratio to random noise emitted by a pixel circuitand a peripheral circuit.

An effective method for this is electron multiplication based on a highelectric field in the interior of a photodiode, and the use of amultiplication-type photodiode such as an APD. That is, the pixel 42 isdesigned as a multiplication-type pixel. Further, on this occasion, ifthe multiplication factor is sufficiently high, also a configuration inwhich an amplification transistor is not provided in the pixel 42 ispossible. Hereinbelow, a configurational example of the pixel 42 in sucha case is shown in FIG. 13 and FIG. 14. Note that, in FIG. 13 and FIG.14, portions corresponding to those in the case of FIG. 9 are markedwith the same reference numerals, and a description thereof is omittedas appropriate.

In the example shown in FIG. 13, the pixel 42 includes the photodiode141 and the reset transistor 147, and the photodiode 141 is designed asa multiplication-type photodiode such as an APD.

In this example, the reset transistor 147 is switched on and thenswitched off, and the pixel 42 is reset to a dark state. That is, thecharge carriers of the photodiode 141 are released to the power source151 via the reset transistor 147. On this occasion, the vertical signalline 153 is connected to the power source 151, and is kept in a floatingstate after the resetting.

Further, a high electric field is applied to the photodiode 141 by usingan impurity profile of the photodiode 141 and the setting of the powersource 151. If photons of a light emission pulse are incident on thephotodiode 141 and a multiplied current flows through the photodiode141, the level (electric potential) of the vertical signal line 153decreases.

In a case where the vertical signal lines 153 of pixels 42 areshort-circuited, the signal generated in the vertical signal line 153 isoutputs of the pixels 42, that is, the sum total of signals inaccordance with charge carriers obtained by the photodiodes 141 of thepixels 42. If a signal obtained by the vertical signal line 153 in thisway and an appropriately set threshold (the comparative electricpotential CV) are compared by the comparator 154, a light emission eventoutput synchronized with the incidence of the light emission pulse, thatis, an event signal can be obtained.

Note that, in a case where the multiplication factor of the photodiodeis particularly high like in an APD that operates in the Geiger mode,the reset transistor 147 may be replaced with a high resistance element.In such a case, the pixel 42 fired with a light emission pulse isautonomously reset to a dark state after a lapse of a certain period oftime.

<Another Configurational Example 2 of Pixel Designed Specially for LightEmission Event Detection>

Further, in the configurational example of the pixel 42 shown in FIG.14, the pixel 42 includes the photodiode 141, the reset transistor 147,and a capacitance element 211, and the photodiode 141 is designed as amultiplication-type photodiode such as an APD.

In this example, the capacitance element 211 is added to a node 212existing between the photodiode 141 and the reset transistor 147, andthe vertical signal line 153 is connected to the anode side of thephotodiode 141.

In the pixel 42, the reset transistor 147 is switched on and thenswitched off, and the pixel 42 is reset to a dark state; thereby, thecapacitance element 211 is charged to the level of the power source 151.That is, charge carriers are accumulated in the capacitance element 211.On this occasion, the electric potential of the vertical signal line 153has been set to, for example, around the ground level, and a highelectric field is applied to the photodiode 141 to set the photodiode141 to a multiplication operation in the Geiger mode.

If photons of a light emission pulse are incident on the photodiode 141and the photodiode 141 comes into conduction, charge carriers charged inthe capacitance element 211 flow into the vertical signal line 153, anda fixed signal is outputted for each pixel 42 regardless of the amountof light (photons) incident on the photodiode 141. That is, depending onthe presence or absence of incidence of photons on each photodiode 141,a binary signal of 1 or 0 indicating this presence or absence isoutputted from each pixel 42 to the vertical signal line 153.

In a case where the vertical signal lines 153 of pixels 42 areshort-circuited, the signal generated in the vertical signal line 153 isin proportion to the total number of pixels 42 fired by light incidence.If a signal obtained by the vertical signal line 153 in this way and anappropriately set threshold (the comparative electric potential CV) arecompared by the comparator 154, a light emission event outputsynchronized with the incidence of the light emission pulse, that is, anevent signal can be obtained.

Note that, also in the pixel 42 shown in FIG. 14, the reset transistor147 may be replaced with a high resistance element. In such a case, thepixel 42 fired with a light emission pulse is autonomously reset to adark state after a lapse of a certain period of time.

Since the pixel 42 of a multiplication-type like the above uses a highelectric field, a large area is occupied for the electrical isolation ofeach element, and it is difficult for the pixel 42 to ensure a highaperture ratio. However, on the other hand, a strong signal can beobtained as the output of the pixel 42 even by small photon incidence.

Usually the aperture ratio of the pixel directly leads to the accuracyof measurement of the amount of light; however, any pixel 42 designedspecially for light emission event detection, like the above pixel 42,has no particular trouble. This is because the amount of light of alight emission pulse can be separately obtained with high accuracy bythe pixel 41 of a non-amplification-type having a high aperture ratio.

<Configurational Example of Light Detector>

Next, a more specific configurational example of the light detector 33described above is described with reference to FIG. 15. The lightdetector 33 has the configuration shown in FIG. 15, for example. Notethat, in FIG. 15, portions corresponding to those in the case of FIG. 5or FIG. 9 are marked with the same reference numerals, and a descriptionthereof is omitted as appropriate.

Note that, in this example, only some of a plurality of pixels 41 and aplurality of pixels 42 are marked with reference numerals, and in FIG.15, the horizontal direction represents the row direction and thevertical direction represents the column direction. That is, a directionorthogonal to the row direction is the column direction.

The light detector 33 shown in FIG. 15 includes a pixel array section241, detection circuits 115, detection circuits 242, switches 243,switches 244, a reference voltage generation circuit 245, a row drivingcircuit 111, a timing control circuit 246, an output circuit 247, anevent detection circuit 248, and an output amplifier 155, and thesecircuits are provided on one chip.

In the pixel array section 241, a plurality of pixels 41 are provided ina two-dimensional lattice configuration. In the following, pixelsarranged in the row direction may be referred to as a pixel row, andpixels arranged in the column direction may be referred to as a pixelcolumn.

In this example, the detection circuit 115, the detection circuit 242,the switch 243, and the switch 244 are provided for each pixel column,and only some of the detection circuits and the switches are marked withreference numerals for easier viewing of the drawing.

In the pixel array section 241, a plurality of pixels 41 included in onepixel column are connected to one detection circuit 115, and similarly aplurality of pixels 41 included in one pixel column are connected to onedetection circuit 242.

Further, in FIG. 15, in order to identify each pixel row, the numericalvalues of “0” to “3” indicating these pixel rows are written on the leftside of the pixel row in the drawing; in the following, each pixel rowis written using these numerical values as appropriate, for examplewritten as the 0th row.

For example, the pixel 41 of the 0th row is connected to the detectioncircuit 115 via a vertical signal line 114, and the pixel 41 of the 1strow is connected to the detection circuit 242 via a vertical signal line251 corresponding to the vertical signal line 114.

Further, each of the pixels 41 is connected to the row driving circuit111 via a control line 252. More specifically, the control line 252includes a plurality of control lines, and these control lines areconnected to the gate of the transfer transistor 103, the gate of thereset transistor 105, and the gate of the selection transistor 107 ofthe pixel 41 shown in FIG. 5, respectively.

Further, in the pixel array section 241, a plurality of pixels 42 areprovided to be arranged in the row direction. Each pixel 42 is connectedto the event detection circuit 248 including the comparator 154 shown inFIG. 9, via a vertical signal line 153.

The output of the event detection circuit 248 is reduced in impedance bythe output amplifier 155, and is outputted as an event signal. Further,each of the pixels 42 is connected to the row driving circuit 111 via acontrol line 253. In a case where, for example, the pixel 42 has theconfiguration shown in FIG. 9, the control line 253 is connected to thegate of the reset transistor 145 and the gate of the reset transistor147 shown in FIG. 9.

The row driving circuit 111 controls each of the pixels 41 in accordancewith control of the timing control circuit 246. The row driving circuit111 PD-resets all the pixels 41 collectively, and causes a new round oflight exposure and accumulation to start. This control corresponds to,for example, the control of timing T1 described with reference to FIG.6.

Further, during readout, the row driving circuit 111 simultaneouslyselects two rows in the column direction, and causes an analogelectrical signal to be outputted to each of the pixels 41 included inthese rows. This electrical signal is read out by the detection circuit115 and the detection circuit 242 and is converted to a digital signal,and an output signal of each pixel 41 is generated.

The two pixel rows selected on this occasion are, for example, pixelrows that are adjacent in the column direction and that include a pixelrow including pixels 41 connected to the detection circuit 115 and apixel row including pixels 41 connected to the detection circuit 242.This control corresponds to, for example, the control of from timing T2to timing T4 described with reference to FIG. 6.

If the readout of the selected two rows is completed, the row drivingcircuit 111 selects the next two rows, and performs similar control. Thereason why readout is performed sequentially in units of two rows inthis way is that each detection circuit 115 and each detection circuit242 are shared between a plurality of pixels 41 arranged in the columndirection.

Further, the row driving circuit 111 performs the resetting of the pixel42 in accordance with control of the timing control circuit 246.

If readout is completed for all the pixel rows including the pixel 41,image data equivalent to one frame, that is, one unit of pulse detection(one unit detection period) are outputted.

For example, in a case where 100 rows×100 columns of pixels 41 areprovided in the pixel array section 241 and each piece of processing oftwo rows takes 16 microseconds (μs), the readout output of one framerequires 50 times of processing and takes approximately 0.8 milliseconds(ms) as a whole.

On this occasion, for example, the detection circuit 115 converts anelectrical signal derived from the pixel 41 of the 0th row to a digitalsignal, and supplies the resulting digital signal to the output circuit247 via the switch 243, in accordance with control of the timing controlcircuit 246.

On the other hand, the detection circuit 242 converts an electricalsignal derived from the pixel 41 of the 1st row to a digital signal, andsupplies the resulting digital signal to the output circuit 247 via theswitch 244, in accordance with control of the timing control circuit246.

More specifically, for example as described with reference to FIG. 6,each of the detection circuit 115 and the detection circuit 242 readsout an electrical signal from the pixel 41 and A/D-converts the signal,generates an output signal from the resulting digital signal, andsupplies the output signal to the output circuit 247 via the switch 243or the switch 244.

The switch 243 opens and closes the path between the correspondingdetection circuit 115 and the output circuit 247. The switches 243provided for the pixel columns become ON and OFF one after another inaccordance with control of a column driving circuit (not illustrated)that selects the pixel columns one after another, and supply outputsignals supplied from the detection circuits 115, to the output circuit247.

The switch 244 opens and closes the path between the correspondingdetection circuit 242 and the output circuit 247. Similarly to theswitches 243, also the switches 244 provided for the pixel columnsbecome ON and OFF one after another in accordance with control of thecolumn driving circuit, and supply output signals supplied from thedetection circuits 242, to the output circuit 247.

The output circuit 247 outputs a digital signal to an image processingapparatus or the like. For example, the output circuit 247 calculates,from the output signals of the pixels 41 supplied from the switches 244and the output signals of the pixels 41 supplied from the switches 243,the sum total of the values of the output signals of all the pixels 41in one frame period (unit detection period), as an output syntheticvalue indicating the amount of light of the light emission pulse, andsupplies the resulting value to the data processing section 22 via thesignal line 43. This output synthetic value is a value indicating theamount of light of the light emission pulse that is incident on thepixel array section 241 from the scintillator 31.

The timing control circuit 246 controls the operating timing of the rowdriving circuit 111, the reference voltage generation circuit 245, thedetection circuit 115, the detection circuit 242, and the eventdetection circuit 248.

For example, the timing control circuit 246 generates a timing controlsignal indicating the scanning timing of the pixel row, and supplies thetiming control signal to the row driving circuit 111. Further, thetiming control circuit 246 generates a digital-to-analog (DAC) controlsignal that controls the operation of supplying a reference voltage, andsupplies the DAC control signal to the reference voltage generationcircuit 245.

Furthermore, the timing control circuit 246 supplies a detection controlsignal that controls the operation of the detection circuit 115 and thedetection circuit 242, to the detection circuit 115 and the detectioncircuit 242. In addition, the timing control circuit 246 performs theresetting of the event detection circuit 248 by a prescribed procedure.

The reference voltage generation circuit 245 generates a referencevoltage Vref to be used for A/D conversion, in accordance with a DACcontrol signal, and supplies the reference voltage Vref to each of thedetection circuits 115 and the detection circuits 242. Furthermore, thereference voltage generation circuit 245 supplies a comparative electricpotential CV to the event detection circuit 248 in accordance with asecond DAC control signal.

For example, in the example shown in FIG. 15, PD resetting is performedand then the light exposure of the pixel 41 is started in each unitdetection period. Further, the output of the pixel 42 is monitoredconstantly by the event detection circuit 248, and a signal that isinputted from the vertical signal line 153 to the comparator 154 in theevent detection circuit 248 in accordance with the output of the pixel42, and the comparative electric potential CV are compared.

Then, the comparison result derived from the comparator 154 is amplifiedby the output amplifier 155 into an event signal, and the event signalis supplied to the data processing section 22 via the signal line 43.

The data processing section 22 detects a light emission event on thebasis of the event signal derived from the output amplifier 155, and ifa light emission event is detected, generates and holds a data object.

That is, if a light emission event is detected, the data processingsection 22 generates a time stamp indicating the time of the generationof the light emission event and stores the time stamp in a data object,and instructs the light detector 33, more specifically, the timingcontrol circuit 246 etc. of the light detector 33 to read out signals.

Consequently, each section of the light detector 33 performs the readoutoperation. On this occasion, the output circuit 247 calculates an outputsynthetic value in one unit detection period from output signalssupplied from the switches 244 and output signals supplied from theswitches 243, and supplies the output synthetic value to the dataprocessing section 22. The data processing section 22 stores, in thedata object, the output synthetic value supplied from the output circuit247 in this way.

Note that, although herein it is described that the output circuit 247calculates an output synthetic value from output signals of the pixels41, the output synthetic value may be calculated by the data processingsection 22. In such a case, the output circuit 247 supplies outputsignals of the pixels 41 to the data processing section 22.

Further, in the configurational example of the light detector 33 shownin FIG. 15, a plurality of pixels 41 are connected to the detectioncircuit 115 or the detection circuit 242, and each of these detectioncircuits is shared between the plurality of pixels 41. However, in acase where the size of the pixel 41 is increased and furthermore thelight detector 33 has a stacked configuration like that described later,each of the detection circuits may be provided immediately below onepixel 41, and the pixel 41 and the detection circuit may be correlatedin a one-to-one manner. In this case, for example, one frame is set to16 μsec, and all the pixels 41 are collectively read out.

As described above, by the radiation counting apparatus 11 to which thepresent technology is applied, a radiation counting apparatus that canperform radiation counting accurately, is small in size and light inweight, is resistant to environmental variations, and can be stablydriven with low voltage and low power consumption can be obtained.Furthermore, in the radiation counting apparatus 11, the pixel 42designed specially for light emission event detection is provided in thelight detector 33 separately from the pixel 41 for measuring the amountof light; thus, also a sufficient time resolution can be ensured duringradiation detection. Therefore, the radiation counting apparatus 11 canbe used also for a nuclear medical diagnosis apparatus that performscoincidence counting, such as a PET, for example.

Second Embodiment <Configurational Example of Radiation CountingApparatus>

Further, in the first embodiment, one scintillator 31 is correlated tothe light detector 33, and scintillation light is diffused over theentire surface of the opening section of the light detector 33. On theother hand, it is also possible to ensure a space resolution on adetection surface (light receiving surface) by correlating ascintillator array to a similar light detector.

In such a case, the radiation counting apparatus has the configurationshown in FIG. 16, for example. Note that, in FIG. 16, portionscorresponding to those in the case of FIG. 1 are marked with the samereference numerals, and a description thereof is omitted as appropriate.

A radiation counting apparatus 281 shown in FIG. 16 includes a lightreceiving section 291 and the data processing section 22. Further, thelight receiving section 291 includes a light detector 301 and ascintillator array 302.

The scintillator array 302 includes four scintillator sections 311-1 to311-4 that are optically separated from each other by a not-illustratedseparating wall, and each scintillator section corresponds to thescintillator 31 shown in FIG. 1. Note that, hereinafter, in a case wherethere is no need to particularly distinguish the scintillator section311-1 to the scintillator section 311-4, they may be referred to assimply a scintillator section 311.

The light detector 301 corresponds to the light detector 33 of FIG. 1,and the opening section, that is, the light receiving surface of thelight detector 301 is separated into four regions C11-1 to C11-4 so asto correspond to the divided scintillator sections 311. Note that,hereinafter, in a case where there is no need to particularlydistinguish region C11-1 to region C11-4, they may be referred to assimply region C11.

Pixels 41 and pixels 42 are provided in each region C11, and the amountof light of a light emission pulse generated in each scintillatorsection 311 is measured by using the pixels 41 in region C11corresponding to the scintillator section 311. That is, in this example,the measurement of the amount of light is performed for each region C11.

Further, pixels 42 are provided in each region C11, and also thedetection of the timing of generation of a light emission pulse, thatis, the detection of a light emission event is performed for each regionC11.

If a light emission event occurs in any one of region C11-1 to regionC11-4, the readout operation is performed selectively on the pixel 41 inregion C11 that is the area where the light emission event has occurred,and an output synthetic value indicating the amount of light of thelight emission pulse is calculated. Then, a data object including theoutput synthetic value and a time stamp is recorded.

Note that, in this example, the resetting of the pixel 41 and the pixel42 may be performed collectively in the pixels 41 and the pixels 42 ofall regions C11.

Further, in the light detector 301, region C11-1 to region C11-4provided on the light receiving surface are separated by a lightblocking section 312, and thereby the leaking-in of light from anot-corresponding scintillator section 311 is prevented.

By employing such a configuration, for example, photons of a lightemission pulse generated by gamma rays that are incident on thescintillator section 311-1 reach only the pixel array section in thecorresponding region C11-1. Thereby, it becomes possible to provide onelight detector 301 with a space resolution; furthermore, a gamma cameraor a PET with an improved space resolution can be obtained by tiling thelight detector 301 and the scintillator array 302.

Note that, although herein an example in which the light receivingsurface of the light detector 301 is divided into regions C11 by thelight blocking section 312 is described, the light blocking section 312may not necessarily be provided.

Third Embodiment <Configurational Example of Light Detector>

Further, the light detector 33 shown in FIG. 1 and the light detector301 shown in FIG. 16 may have a stacked configuration. For example, in acase where the light detector 33 has a stacked configuration, the lightdetector 33 has the configuration shown in FIG. 17. Note that, in FIG.17, portions corresponding to those in the case of FIG. 15 are markedwith the same reference numerals, and a description thereof is omittedas appropriate.

The light detector 33 shown in FIG. 17 includes one chip obtained bystacking an upper-side substrate 341 and a lower-side substrate 342.

The pixel array section 241 is provided in the upper-side substrate 341,and pixels 41 and pixels 42 are provided to be arranged in the pixelarray section 241, as described with reference to FIG. 15.

Further, the detection circuit 115, the detection circuit 242, the eventdetection circuit 248, the row driving circuit 111, the referencevoltage generation circuit 245, the timing control circuit 246, theoutput circuit 247, etc. are provided in the lower-side substrate 342.Note that, although herein only some of the detection circuits 115 andthe detection circuits 242 are marked with reference numerals, aplurality of detection circuits 115 and a plurality of detectioncircuits 242 are provided in the lower-side substrate 342.

The upper-side substrate 341 and the lower-side substrate 342 arestacked by silicon stacking technology such as adhesion of siliconwafers, for example.

In this example, the scintillator 31 is located on the upper-side in thedrawing with respect to the pixel 41 and the pixel 42 of the pixel arraysection 241, and photons of a light emission pulse are incident on thepixel 41 and the pixel 42 from the upper side in the drawing. Thus, thelower-side substrate 342 in which each circuit is provided is stacked onthe opposite side to the light incidence side of the pixel array section241; thereby, the light receiving surface, that is, the aperture ratioof each pixel of the pixel array section 241 can be improved.

As a result, even if a large-sized scintillator 31 is connected to thelight detector 33 in order to increase radiation sensitivity, most ofthe light emission pulses generated in the scintillator 31 can bereceived.

In particular, in a case where tiling is performed in the manner of theexample shown in FIG. 16, that is, in a case where the light receivingsurface is divided into several regions and radiation counting isperformed independently in each region, the yield of light emissionpulses can be improved. Also in the use example shown in FIG. 17,minimizing the fringe section other than the opening section includingthe pixel 41 and the pixel 42 enables tiling in an uniform opening,while narrowing the width of a light blocking section corresponding tothe light blocking section 312 of FIG. 16.

The quantum efficiency of such a large-sized semiconductor pixel isnearly 100%, and therefore the energy resolution is equivalent to aphotomultiplier in many uses. Further, semiconductor light detectors canbe mass-produced in the same manufacturing line as and by a similarmanufacturing process to CMOS image sensors on the market.

The radiation counting apparatus 11 including the light detector 33 thusmanufactured is small in size and light in weight, is resistant toenvironmental variations, has stable characteristics, and is easy tomaintain. Further, the output of the light detector 33 is a digitalsignal; thus, the circuit required in later stages may be only a circuitof processing of digital signals, and not only is the output lesssusceptible to noise from the surroundings, but also data outputted froma large number of light receiving sections can be easily processed.

By providing the pixel 41 and the pixel 42, and circuits such as thedetection circuit 115 and the detection circuit 242 in differentsubstrates and stacking the substrates in the above manner, the ratio ofthe area of the pixel array section 241 in the light receiving surfaceof the light detector 33 (aperture ratio) can be increased, and theenergy resolution can be improved.

Fourth Embodiment <With Regard to Example of Application to FlowCytometry>

Meanwhile, the light detector 33 to which the present technology isapplied can also be used for other fields than radiation counting. Forexample, flow cytometry is given as an example of acquisition of afeeble fluorescence pulse or the like by the light detector 33 in otherfields than radiation counting.

Hereinbelow, an example in which the present technology is applied toflow cytometry is described with reference to FIG. 18. Note that, inFIG. 18, portions corresponding to those in the case of FIG. 1 aremarked with the same reference numerals, and a description thereof isomitted as appropriate.

In the example shown in FIG. 18, test samples 372 such as cells passedfrom a sample tube 371 are arranged in a line in a sample flow 373, andthis place is irradiated with laser light emitted from a light source374. If a test sample 372 passes through an irradiation spot 375 oflaser light, scattered light and fluorescence excited from afluorescence marker or the like are generated.

Then, front scattered light, which has a larger amount of light, isreceived by a photodiode 376, and the size of the test sample 372 isdetected.

On the other hand, side scattered light or fluorescence emitted by afluorescence marker attached to the test sample 372 becomes feeble pulselight, and is sensed by the light detector 33 functioning as a pulselight detector. Information regarding the type and internal structure ofthe test sample 372 is acquired by the fluorescence and the sidescattered light.

Here, a manner of feeble pulse detection is shown in FIG. 19. Note that,in FIG. 19, the horizontal direction represents the time direction.Further, the curved line indicated by arrow W11 represents the intensityof light that is incident on the light detector 33 from the test sample372, that is, side scattered light or fluorescence at each time.Furthermore, the curved line indicated by arrow W12 represents theintensity of light that is incident on one pixel 42 of the lightdetector 33 from the test sample 372, that is, side scattered light orfluorescence at each time.

In association with the passage of a test sample 372 through theirradiation spot 375 of laser light, the intensity of side scatteredlight or fluorescence has a pulse shape like the portion indicated byarrow W13 of the curved line indicated by arrow W11. Each pulse portionof the curved line indicated by arrow W11 corresponds to the passage ofone test sample 372 through the irradiation spot 375.

From an integrated signal of pulse light W14 that is incident on thepixel 42 and is included in side scattered light or fluorescence of theportion indicated by arrow W13, the pixel 42 designed specially forlight emission event detection included in the light detector 33 outputsan event signal EV11 almost synchronized with the timing of passage ofthe test sample 372, at timing T31.

Here, the completion of light exposure of the pixel 41 for measuring theamount of light and the readout of an output in the light detector 33are performed in synchronization with the event signal associated withthe passage of the test sample 372 through the irradiation spot 375.

Specifically, the light detector 33 performs the driving described withreference to FIG. 8, that is, global shutter driving in which there ispractically no dead period of side scattered light or fluorescence.

On this occasion, charge carrier transfer in the pixel 41 is performedin synchronization with the event signal EV11 of test sample passage,and the light exposure period ends and readout is started. Further, thenext light exposure period is started in all the pixels 41 all at once.

That is, at timing T32 at which a certain delay that takes the flowvelocity and size of the test sample 372 into consideration has elapsedafter the event signal EV11 is acquired, the light detector 33 completeslight exposure (accumulation) in each pixel 41 for measuring the amountof light, and starts the readout of accumulation signals from the pixel41.

Here, timing T32 corresponds to timing T13 in FIG. 8. The light exposureperiod is caused to end after a certain period of time from when theevent signal EV11 is acquired, that is, from when the light emissionevent is detected, and thereby it becomes possible to receive, with thepixel 41, light emitted from the test sample 372, that is, all the lightof the portion indicated by arrow W13.

Further, on starting the readout of accumulation signals, the lightdetector 33 starts the next round of light exposure (accumulation) ofthe pixel 41. Note that the readout (sampling) of reset signals may beperformed during the period from when the immediately preceding readoutof accumulation signals ends and before the light exposure period ends.

An output synthetic value that is the total value of outputs of thepixels 41, that is, the sum total of output signals of the pixels 41 ineach readout sequence is equivalent to the total amount of photonsreceived by the light detector 33 for each piece of pulse light that isside scattered light or fluorescence. Thereby, the intensity of sidescattered light or fluorescence for each test sample 372 is derived.

Note that, in a case where, like in a flow cytometer, test samples flowalmost continuously and light pulses are generated continuously atintervals of, although fluctuating, less than or equal to 100 μsec,there is little room for the periodic resetting shown in FIG. 4 tooccur. Thus, periodic resetting of the pixel may be omitted from thesequence of detection.

Also in a case where the present technology is used for fluorescencedetection in flow cytometry or the like in the above manner, theapparatus can be downsized, and furthermore stable operation resistantto environmental variations can be achieved. Further, a sufficient timeresolution for light emission event detection can be ensured.

Fifth Embodiment <Configurational Example of Light Detector>

Meanwhile, the method for arranging pixels used for light emission eventdetection has various variations; for example, a pixel for lightemission event detection such as the pixel 42 may be provided separatelyfrom a pixel array including pixels for measuring the amount of lightsuch as the pixel 41 described above, independently around the pixelarray.

Further, a pixel normally used to measure the amount of light may beused to be switched to a pixel for event detection by a switchmechanism. In such a case, the light detector 33 has the configurationshown in FIG. 20, for example. Note that, in FIG. 20, portionscorresponding to those in the case of FIG. 15 are marked with the samereference numerals, and a description thereof is omitted as appropriate.

In the light detector 33 shown in FIG. 20, a plurality of pixels 41 arearranged as an array in the row direction and the column direction inthe pixel array section 241, and a plurality of pixels 41 arranged inthe column direction are connected to one vertical signal line 114. Notethat each of these pixels 41 has, for example, the circuit configurationshown in FIG. 5.

An input terminal of a switch 401 is connected to the vertical signalline 114, and the switch 401 switches its output destination to eitherone of the detection circuit 115 and the event detection circuit 248, asnecessary. That is, an input terminal of the switch 401 is connected tothe vertical signal line 114, the detection circuit 115 is connected toone output terminal of the switch 401, and the event detection circuit248 is connected to another output terminal of the switch 401.

For example, in a case where there is no need to monitor a lightemission event, that is, in a case where there is no need to perform thedetection of a light emission event, the switch 401 is fixed to remainconnected to the output terminal on the detection circuit 115 side, andan output of the pixel 41 is supplied to the detection circuit 115 viathe vertical signal line 114 and the switch 401. Then, the readout ofaccumulated charge carriers from the pixel 41 is performed periodicallyon a pixel row basis by the detection circuit 115.

On the other hand, in a case where it is intended to perform readoutafter detecting light incidence, that is, in a case where the monitoringof a light emission event is performed, a switching operation of theswitch 401 like below is performed.

In this case, for example, a light emission event is detected by onepixel row 402 including pixels 41. That is, the pixel 41 included in thepixel row 402 functions as a pixel for light emission event detection.

First, the photodiodes 101 of the pixels 41 are reset all at once; whilethe presence or absence of incidence of a light emission pulse ismonitored, all the pixels 41 included in a specific pixel row 402 areselected as a pixel used for light emission event detection all thetime. That is, the transfer transistor 103 and the selection transistor107 of each pixel 41 of the pixel row 402 are controlled to the ON stateby the row driving circuit 111.

Further, simultaneously with this, the switch 401 is connected to theoutput terminal on the event detection circuit 248 side, and an outputof each pixel 41 of the pixel row 402 is supplied to a comparator forevent monitoring, that is, the event detection circuit 248 via thevertical signal line 114 and the switch 401. Then, the outputs of thesepixels 41 are continuously monitored by the event detection circuit 248.

If a light emission event is detected by the event detection circuit248, the light detector 33 controls the switch 401 so that the switch401 is connected to the output terminal on the detection circuit 115side, and that an output of each pixel 41 is supplied to the detectioncircuit 115 via the vertical signal line 114 and the switch 401.

Then, the row driving circuit 111 sequentially selects all the otherpixel rows than the pixel row 402 that has been used for the detectionof a light emission event, and causes outputs from the pixels 41 ofthese pixel rows set to a selection state to be supplied to thedetection circuit 115.

The detection circuit 115 A/D-converts a signal read out from each pixel41, and supplies, to a not-illustrated output circuit 247, the resultingdigital output signal indicating the amount of light of photons incidenton the pixel 41. Further, the output circuit 247 calculates an outputsynthetic value indicating the amount of light of the light emissionpulse, on the basis of the output signal of each pixel 41 supplied fromthe detection circuit 115.

An advantage of the configuration of such a light detector 33 shown inFIG. 20 is that the same chip can be used for both normal imaging andlight emission event detection in accordance with the use.

Furthermore, there is also an advantage that a pixel 41 in a desiredplace can be used as a pixel for light emission event detection by theselection of the switch 401 and the row driving circuit 111. Thereby, itis also possible to perform the detection of a light emission eventwhile avoiding a defective pixel with a very large dark current; thus,there is also an advantage that false detection of a light emissionevent due to a defective pixel can be reduced.

Note that the embodiment of the present technology is not limited to theembodiments described above, and various alterations are possiblewithout departing from the spirit of the present technology.

Additionally, the present technology may also be configured as below.

-   (1)

An imaging element including:

a pixel array section including

-   -   a plurality of first pixels each including        -   a first photoelectric conversion section configured to            photoelectrically convert incident light,        -   a floating diffusion section configured to generate a            voltage in accordance with an amount of charge carriers            obtained by the photoelectric conversion, and        -   a transfer section configured to transfer the charge            carriers from the first photoelectric conversion section to            the floating diffusion section, and    -   a second pixel configured to detect incidence of light,    -   in which readout of a signal is performed intermittently from        each of the plurality of first pixels in accordance with the        voltage, and    -   an output of the second pixel is monitored continuously.

-   (2)

The imaging element according to (1), further including:

an event detection section configured to output a signal synchronizedwith incidence of light on the second pixel, on a basis of the output ofthe second pixel.

-   (3)

The imaging element according to (2), further including:

a driving section configured to control readout of a signal from thefirst pixel in accordance with an output of a signal from the eventdetection section.

-   (4)

The imaging element according to (3),

in which the driving section controls the transfer section in accordancewith an output of a signal from the event detection section to cause thecharge carriers obtained by the photoelectric conversion to betransferred to the floating diffusion section, and controls readout of asignal from the first pixel.

-   (5)

The imaging element according to any one of (1) to (4), furtherincluding:

-   -   a detection section configured to generate a digital signal        indicating an amount of light incident on the first pixel, on a        basis of a signal read out from the first pixel.

-   (6)

The imaging element according to (5), further including:

an output section configured to calculate an amount of light incident onthe pixel array section, on a basis of the digital signal obtained foreach of the plurality of first pixels.

-   (7)

The imaging element according to (5) or (6),

in which the first pixel further includes

-   -   a first amplification section configured to output a signal in        accordance with the voltage generated by the floating diffusion        section, and    -   a selection section configured to enter a conduction state or a        non-conduction state in accordance with control and, on entering        the conduction state, output a signal outputted from the first        amplification section, to the detection section.

-   (8)

The imaging element according to any one of (1) to (7),

in which the second pixel includes

-   -   a second photoelectric conversion section configured to        photoelectrically convert incident light, and    -   a second amplification section configured to output a signal in        accordance with charge carriers obtained by the photoelectric        conversion by the second photoelectric conversion section.

-   (9)

The imaging element according to any one of (1) to (7),

in which the first pixel is a non-multiplication-type pixel, and thesecond pixel is a multiplication-type pixel.

-   (10)

The imaging element according to any one of (1) to (9),

in which a light receiving surface of the second pixel is larger than alight receiving surface of the first pixel.

-   (11)

A driving method for driving an imaging element,

the imaging element including

a pixel array section including

-   -   a plurality of first pixels each including        -   a photoelectric conversion section configured to            photoelectrically convert incident light,        -   a floating diffusion section configured to generate a            voltage in accordance with an amount of charge carriers            obtained by the photoelectric conversion, and        -   a transfer section configured to transfer the charge            carriers from the photoelectric conversion section to the            floating diffusion section, and    -   a second pixel configured to detect incidence of light,

the driving method including:

a step of continuously monitoring an output of the second pixel anddetecting incidence of light on the second pixel on a basis of theoutput of the second pixel; and

a step of resetting the first pixel periodically, and controllingreadout of a signal according to the voltage from the first pixel, inaccordance with detection of incidence of light on the second pixel.

-   (12)

An electronic device including:

a pixel array section including

-   -   a plurality of first pixels each including        -   a photoelectric conversion section configured to            photoelectrically convert incident light,        -   a floating diffusion section configured to generate a            voltage in accordance with an amount of charge carriers            obtained by the photoelectric conversion, and        -   a transfer section configured to transfer the charge            carriers from the photoelectric conversion section to the            floating diffusion section,    -   a second pixel configured to detect incidence of light,    -   in which readout of a signal is performed intermittently from        each of the plurality of first pixels in accordance with the        voltage, and    -   an output of the second pixel is monitored continuously.

REFERENCE SIGNS LIST

-   11 radiation counting apparatus-   21 light receiving section-   22 data processing section-   31 scintillator-   33 light detector-   41 pixel-   42 pixel-   101 photodiode-   103 transfer transistor-   104 detection node-   106 amplification transistor-   111 row driving circuit-   115 detection circuit-   141 photodiode-   146 amplification transistor-   154 comparator

1. An imaging element comprising: a pixel array section including aplurality of first pixels each including a first photoelectricconversion section configured to photoelectrically convert incidentlight, a floating diffusion section configured to generate a voltage inaccordance with an amount of charge carriers obtained by thephotoelectric conversion, and a transfer section configured to transferthe charge carriers from the first photoelectric conversion section tothe floating diffusion section, and a second pixel configured to detectincidence of light, wherein readout of a signal is performedintermittently from each of the plurality of first pixels in accordancewith the voltage, and an output of the second pixel is monitoredcontinuously.
 2. The imaging element according to claim 1, furthercomprising: an event detection section configured to output a signalsynchronized with incidence of light on the second pixel, on a basis ofthe output of the second pixel.
 3. The imaging element according toclaim 2, further comprising: a driving section configured to controlreadout of a signal from the first pixel in accordance with an output ofa signal from the event detection section.
 4. The imaging elementaccording to claim 3, wherein the driving section controls the transfersection in accordance with an output of a signal from the eventdetection section to cause the charge carriers obtained by thephotoelectric conversion to be transferred to the floating diffusionsection, and controls readout of a signal from the first pixel.
 5. Theimaging element according to claim 1, further comprising: a detectionsection configured to generate a digital signal indicating an amount oflight incident on the first pixel, on a basis of a signal read out fromthe first pixel.
 6. The imaging element according to claim 5, furthercomprising: an output section configured to calculate an amount of lightincident on the pixel array section, on a basis of the digital signalobtained for each of the plurality of first pixels.
 7. The imagingelement according to claim 5, wherein the first pixel further includes afirst amplification section configured to output a signal in accordancewith the voltage generated by the floating diffusion section, and aselection section configured to enter a conduction state or anon-conduction state in accordance with control and, on entering theconduction state, output a signal outputted from the first amplificationsection, to the detection section.
 8. The imaging element according toclaim 1, wherein the second pixel includes a second photoelectricconversion section configured to photoelectrically convert incidentlight, and a second amplification section configured to output a signalin accordance with charge carriers obtained by the photoelectricconversion by the second photoelectric conversion section.
 9. Theimaging element according to claim 1, wherein the first pixel is anon-multiplication-type pixel, and the second pixel is amultiplication-type pixel.
 10. The imaging element according to claim 1,wherein a light receiving surface of the second pixel is larger than alight receiving surface of the first pixel.
 11. A driving method fordriving an imaging element, the imaging element including a pixel arraysection including a plurality of first pixels each including aphotoelectric conversion section configured to photoelectrically convertincident light, a floating diffusion section configured to generate avoltage in accordance with an amount of charge carriers obtained by thephotoelectric conversion, and a transfer section configured to transferthe charge carriers from the photoelectric conversion section to thefloating diffusion section, and a second pixel configured to detectincidence of light, the driving method comprising: a step ofcontinuously monitoring an output of the second pixel and detectingincidence of light on the second pixel on a basis of the output of thesecond pixel; and a step of resetting the first pixel periodically, andcontrolling readout of a signal according to the voltage from the firstpixel, in accordance with detection of incidence of light on the secondpixel.
 12. An electronic device comprising: a pixel array sectionincluding a plurality of first pixels each including a photoelectricconversion section configured to photoelectrically convert incidentlight, a floating diffusion section configured to generate a voltage inaccordance with an amount of charge carriers obtained by thephotoelectric conversion, and a transfer section configured to transferthe charge carriers from the photoelectric conversion section to thefloating diffusion section, a second pixel configured to detectincidence of light, wherein readout of a signal is performedintermittently from each of the plurality of first pixels in accordancewith the voltage, and an output of the second pixel is monitoredcontinuously.